Metal-oxide-semiconductor (MOS) transistors incorporate a metal or conductive polysilicon gate which, when energized, inverts an underlying channel region between a source region and a drain region. The inverted channel region then acts as an ohmic path between the source and drain regions for conducting current across the MOS transistor. The amount of charge on the gate determines the conductivity of the channel region, and, in certain cases, it is desirable to quickly remove charge (positive or negative) from the gate to quickly turn the transistor off. If charge were allowed to remain on the gate after a voltage supply has been disconnected from the gate, this residual charge will cause the transistor to remain on for a certain period of time until the charge on the gate leaks off due to some parasitic resistance.
One way to discharge the gate of an MOS transistor is to switch the gate to a reference voltage (e.g., ground for an NMOS transistor) immediately after a driving voltage supply has been disconnected from the gate. The switching mechanism used to discharge the gate is usually another transistor connected between the gate and the reference voltage. This discharging transistor frequently requires additional masking steps to form and requires some control circuitry for switching the discharging transistor at the proper time.
Another known way to remove residual charge from the gate of an MOS transistor is to provide a large value resistor between the gate and a reference voltage to act as a weak pull-down device (for an NMOS transistor) or pull-up device (for a PMOS transistor). When a driving voltage is applied to the gate to turn the transistor on, only a small leakage current flows through the high value resistor. When the driving voltage is removed from the gate, the small residual charge on the gate is quickly removed by the resistor. High value leakage resistors are commonly formed by MOS depletion transistors having a long and narrow gate to achieve the high resistance value, but these resistors require additional masking steps.
The high value resistor is frequently provided as a discrete device (not on the same chip as the transistor) which is connected between the gate of an MOS power transistor and a reference voltage terminal. These discrete devices take up printed circuit board area, require manufacturing time to interconnect the resistor between the gate and the reference voltage terminal, and are relatively expensive.
What is needed is a high value leakage resistor for an MOS transistor which takes up little area and can be formed on the same wafer as the MOS transistor without using any additional masking steps. Ideally, this resistor can be hidden or buried under the transistor's non-active areas, such as the bonding pads or metal interconnects, and hence, not require any additional die area.